U.S. Pat. No. 4,806,893 entitled "Molded Case Circuit Breaker Actuator-Accessory Unit" describes the use of an electromagnetic actuator within an actuator-accessory unit to articulate a circuit breaker operating mechanism to separate the circuit breaker contacts upon the occurrence of an overcurrent condition of predetermined magnitude. An electronic trip unit in combination with current transformers are used within so-called "electronic trip" circuit breakers to sense the circuit current and determine when such a tripping function should be inputted to the actuator-accessory unit. The electronic trip unit and actuator-accessory unit replace prior art thermally and magnetically active trip elements which respond to changing circuit current in an analog fashion.
The operating mechanism and latch assembly used within the electronic trip circuit breakers are described in U.S. Pat. No. 4,736,174 entitled "Molded Case Circuit Breaker Operating Mechanism" and U.S. Pat. No. 4,864,263 entitled "Molded Case Circuit Breaker Latch and Operating Mechanism Assembly".
The "tripping force" for purposes of this disclosure is defined as the amount of force required to displace the operating mechanism latch from the operating cradle to allow the operating springs to overcenter and separate the circuit breaker contacts.
With higher ampere-rated circuit breakers, a higher tripping force is generally required to overcome the higher latching forces generated between the operating cradle and the latch assembly. U.S. Pat. application Ser. No. 518,673 filed May 3, 1990 and entitled "Tripping Arrangement for Molded Case Circuit Interrupter" describes a supplemental tripping assembly for providing additional trip force to one such higher ampere-rated industrial circuit breaker. Another approach to compensate for the higher trip forces required with higher ampere-rated industrial circuit breakers is to provide a supplemental latch in combination with the latch assembly which effectively reduces the latching forces. One such supplemental latch is found within U.S. Pat. application Ser. No. 526,481 filed May 21, 1990 and entitled "Molded Case Circuit Breaker Compact Latch Assembly".
U.S. Pat. application Ser. No. 582,683 filed Sep. 14, 1990 entitled "Molded Case Circuit Breaker Operating Mechanism Cradle Configuration" describes an optimum latch configuration that requires a reduced tripping force to displace the operating cradle from the circuit breaker latch assembly. All of the aforementioned U.S. Patents and Patent Applications are incorporated herein for reference purposes.
To economically manufacture an operating cradle having the configuration described within the aforementioned U.S. Pat. application Ser. No. 582,683 the metal pieces are formed by means of an inexpensive punch and die process that requires no further grinding or polishing operations. To provide a durable surface, the cradle is hardened in a tumbler within a high temperature furnace, the steel composition must accordingly be selected to prevent the cradle geometry from becoming distorted during the tumbling operation.
The instant invention describes an optimum steel composition for fabricating the cradles without distorting the cradle geometry and which provide a durable cradle operating surface over long periods of continuous operation within an industrial-rated circuit breaker.